The effect of alpha-tocopherol on the oxidative transformation of arachidonic acid was investigated in human platelets. The major products of lipoxygenase and cyclo-oxygenase pathways were separated by high performance liquid chromatography (HPLC) and thin layer chromatography (TLC) evaluated by scanning the radiochromatograms. This study differs from others in the vitamin E field in important aspects of its experimental design: the prelabeling of platelets with non-aggregating concentrations of 14C-arachidonic acid, and the addition of alpha-tocopherol as a colloidal suspension rather than as an ethanolic solution. A moderately potent but consistent reduction of apparent cyclo-oxygenase activity by alpha-tocopherol could be demonstrated by TLC and HPLC. This effect was best shown by the change of the HETE/HHT ratio which increased significantly in vitamin E-treated platelets. It was found to be dose-dependent up to 1 microM alpha-tocopherol, the maximal concentration tested in this study. Alpha-tocopherol quinone was equally effective in this action.